High intensity discharge (HID) lamps are highly desirable for commercial markets due to their superior light efficiency in terms of lumens produced per watt and the variety of power ranges in which they are available. As HID lamp technology has developed, manufacturers and ballast designers have come to focus on reliability and efficiency in providing a robust and simple HID lighting system.
U.S. Pat. No. 7,078,870, which is assigned to the assignee of the present application, International Rectifier Corporation, discloses an example of a ballast circuit for use in driving an HID lamp. The entire contents of U.S. Pat. No. 7,078,870 are hereby incorporated by reference herein. The ballast system disclosed in this reference, however, fails to take into account the behavior of the HID lamp immediately after ignition and in the warm up phase. When high voltage is applied across an HID lamp, the lamp produces an arc which will initially draw a very large current, resulting in lamp voltage dropping as low as 20V or less. Immediately after ignition, most of the energy stored in the series capacitors C1, C2 to which the lamp is returned is discharged into the lamp. If the amount of energy is too small, or too great, the lamp will extinguish immediately after ignition such that the result is merely merely a flash.
Using the system described in U.S. Pat. No. 7,078,870, the capacitors C1, C2 would have to be very small in order to prevent too much energy from being discharged into the lamp. However, the lamp current immediately after ignition must be maintained at a sufficiently high level to keep the lamp lit. This would be impossible using such small capacitors.
Thus, it would be desirable to provide ballast circuit system that avoids these problems.